Chinh Dang

Electrical aging of extruded dielectric cables. A physical model

A model is proposed to describe all experimental results on electrical aging of cables reported in Part 1. It is based on simple thermodynamics concepts in the Eyring theory, includes the concept of submicrocavity formation proposed by Zhurkov, and supposes that the first step in electrical aging is essentially a molecular process, as in Crine and Vijhs model. Our model of electrical aging under ac fields supposes that molecular-chain deformation is essentially a fatigue process and, therefore, that high frequencies generate more defects and thus reduce cable life, as indeed demonstrated by others. An original feature of the model is the submicrocavity formation above a critical field F/sub c/, whose value can be approximately predicted knowing the energy of cohesion of the polymer. This leads to a simple lifetime equation depending on just two physical parameters /spl Delta/G/sub 0/ (energy of activation of the chain deformation process) and /spl lambda//sub max/ (the maximum size of submicrocavities) with no adjustable unknowns. Above F/sub c/, there is an exponential relation between time and field, whereas below F/sub c/, the breakdown strength of the insulation varies very little with time; in other words, there is very limited (if any) aging. The slope of the exponential regime gives the value of /spl lambda//sub max/ directly whereas the intercept gives the value of /spl Delta/G/sub 0/. The predictions made by the model are discussed in correlation with existing experimental data. In addition to these basic assumptions, the model confirms that there is a relation between cable endurance and insulation morphology. Actually, the size of submicrocavities is ultimately limited by the amorphous-phase thickness. The max values deduced from the slopes of the exponential regime between F and log t for polyethylene (PE) (Part 1), XLPE and EPR insulation are in excellent agreement with the size of the amorphous phase of these samples, as measured by X-ray spectroscopy. It is also shown that the presence of water results in a lower /spl Delta/G/sub 0/ value, i.e. a shorter life. The precise relation between /spl Delta/G/sub 0/ and the nature and concentration of the impurity (including water) needs more work. The impact of these conclusions on the experimental limits of a reliable accelerated aging test and on the final breakdown process are discussed in a subsequent paper.

A new approach to the electric aging of dielectrics

A simple model partially based on the rate theory is proposed to describe the electric aging of polyethylene (PE) and cross-linked PE (XLPE) cable insulation. It is assumed that the stress (electric or mechanical) reduces the height of the energy barrier controlling the process under study. The model describes very different effects of high electric fields on the properties of polymers. It relies on two parameters whose physical significance is discussed in the light of electric aging of PE and polarization reversal of PVDF. A preliminary study indicates that very long lifetimes of polymers can be achieved provided the applied field is lower than the critical field controlled by the chemical and morphological structure of the material.<<ETX>>

Interfacial breakdown in cable joints

Electrical breakdown at the interface of two dielectric surfaces is a complex phenomenon and is one of the major causes of failure for cable accessories. The dielectric strength of an interface increases with interfacial pressure and is substantially improved by the presence of electrical insulating greases. In order to better understand this phenomenon, interfacial breakdown experiments were carried out on interfaces found in cable joints. Measurements of breakdown voltage, interfacial pressure and grease conductivity were made to provide a comprehensive set of data for the investigation. This paper describes the experimental approach and discusses the preliminary results of the investigation on breakdown voltages found in new and field-aged cable splices.<<ETX>>

Metallurgic and contact resistance studies of sleeve connectors in aluminium cable splices

Two types of sleeve connector (crimp and solder) used in aluminum cable splices were examined after being tested in a series of accelerated aging experiments. Diagnostic techniques, such as optical microscopy, scanning electron microscopy, and energy dispersive X-ray analysis, were applied to reveal the microstructure of the contact. The observation correlates well with the measurements of the contact resistance. Corrective measures are suggested to improve the performance of the cable splices.<<ETX>>

A study of the interfacial breakdown in cable joints

This paper examines the interchangeability of cable adapters in premolded T-connectors from the standpoint of their interfacial dielectric performance based upon measurements of the breakdown voltage and the interfacial pressure. The design differences between cable adapters vary from materials composition to small dimensional changes. As a result, the dielectric strength at interfaces between the cable, the cable adapter and the connector housing is affected. Furthermore, aging also reduces the interfacial pressure as well as the dielectric property of grease at the interfaces. A reduction in dielectric strength could initiate the development of partial discharges and eventually lead to the dielectric breakdown in service.

Effect of surface conditions on the breakdown strength of various dielectric interfaces

Results of this study have shown that the breakdown strength greatly depends on the type of dielectrics forming an interface as well as their surface conditions. Some greases perform much better than others in maintaining the dielectric integrity of the interface; this is so despite the change in substrate materials, and the sliding as well as the sanding in assemblage. On the contrary, the high-K layer material tested is not effective in controlling the interface performance; it produces little improvement on the dielectric strength of an interface, however it does reduce significantly the dispersion of the breakdown voltage. An interface with a high-K layer is both affected by the substrate material and the sanding.

An effective-resistance measurement technique for improving the current-cycling test of power connectors

The current-cycling tests as prescribed by the ANSI C119.4 and CSA-C57 standards have been in use for many years by utilities in the US and Canada to evaluate the performance of electrical connectors. Although these standards can provide some satisfactory level of assurance for the users, their evaluation criteria are, however, not sensitive enough to detect the contact degradation. As a consequence, a great variety of commercially available connectors can conform to the acceptance criteria setup by these standards, but show problematic performance in the field applications. This paper presents a new effective-resistance measurement technique which significantly improves its sensitivity to the contact deterioration. Application of this technique in the current-cycling testing proved to be a valuable diagnostic tool in evaluating and rating the connector performance.

Compression connector performance on oxidized aluminum cable conductors

Oxidized aluminum conductors are often found in overhead and underground distribution systems. Jointing these oxidized aluminum conductors with compression connectors in cable joints can lead to thermal runaway and fire hazard as found in our laboratory experiments. This problem was partly alleviated by the clearing and degreasing processes used in the rejuvenation of the underground cable insulation by silicone injection. It can, however, be eliminated by chemical cleaning.

Underground Medium-Voltage Cable Rejuvenation—Part III: Influence of Operating and Emergency Conditions

The present study, the last of three parts, deals with the testing of three solutions for the blocking of cable-ends of silicone injected cables on an underground experimental line at up to maximum operating conditions. Another objective is to verify in the laboratory the impact of emergency conditions on silicone-injected cables. The experimental line is composed of 16 triplexed XLPE cable sections, rated 28-kV, having 750-kcm aluminum conductors; the line comprises also 42 splices. Only two phases of each cable section were silicone-injected by a service provider. It was found that the solution to dam cable-ends after silicone injection performs successfully and, also, that the silicone diffuses rapidly in the cables extruded layers. It was further revealed that, with freshly injected cables problems arise due to high conductor temperature above 100°C, but it becomes harmless a few months after injection.

Performance of compression connectors on strand-filled conductors

Water-related damages are of major concern to the reliability of underground lines. Several strategies are developed to prevent water entry in extruded cables. Strand-filled cables are one of the available means to prevent water penetration and circulation in underground cable conductors, thus eliminating aluminum conductor corrosion and hydrogen gas buildup as well as reducing water-related treeing. However, published results on the connection performance with strand-filled aluminum conductors were scant and inconclusive. This report summarizes our latest findings on the subject with more conclusive data and some unexpected results.